Resistor



L Forsen,

. :QS t-Jor- Delaware The present invention relates to resistors and more particularly to low inductance resistors having a pulsed current capacity above about 1,090 ainperes.

Presently available low inductance resistors have been designed for relatively low current. When such a resistor design has been employed to construct a resistor for high currents, especially greater than 1,000 amperes, the resulting resistor has been torn apart by the magnetic forces developed in the resistor and/or has been heated to such a high temperature that the resistor has burned out.

An object of the present invention is the provision of a low inductance resistor having a pulsed current capacity of over about 1,060 arnperes. Another object of the invention is the provision of a low inductance resistor which has a relatively long life with current pulses of over LOllO arnpercs. Still another object is the provision of a low inductance high current resistor which is durable in use and inexpensive to manufacture Various other objects and advantages of the present invention will become apparent by reference to the following description and accompanying drawings.

in the drawings:

PEGURE l is a perspective view of a low inductance high current resistor constructed in accordance with the present invention.

FIGURE 2 is an enlarged side View of the resistor shown in FlGURE 1; and

FEGURE 3 is a cross sectional view taken generally along line 33 of FIGURE 2.

in accordance with the present invention a low inductance resistor is provided which has a pulsed current capacity above approximately 1,000 amperes. Gen orally the resistor comprises an elongated ribbon of resistive material which is bent back upon itself so as to form a pair of spaced apart, coextensively extending folded back sections. A spacer of insulating material is disposed between the folded back s ctions of the ribbon and an element of insulating material is disposed on the outwardly facing surface of each section. Means are provided for clamping the spacer, the folded back sections and the elements together with a pressure such that the frictional force on the sections is greater than the force due to t errnal expansion of the sections.

More specifically, the resistor illustrated in the drawing includes an elongated ribbon lid of resistance mate rial, such as stainless steel, Nichrome, Monel, Advance metal (an alloy of 43% nickel and the balance copper), etc. The ribbon it is made of a cross sectional area and length which depend upon the resistance and inductance desired.

The ribbon lit is folded over on itself through a series of alternately reverse bends extending across the width f the ribbon so as to provide an even number of spaced apart, coextensively extending parallel sections or folds l2 interconnected by portions 13. In this way, the magnetic field energy produced by the current flowing through one section 12 is substantially reduced by the magnetic field produced by the oppositely directed current in the other section 32 of the pair. Accordingly, a low inductance results. The closer the spacing etween the sections 12 the lower is the inductance of the resistor. However, as the sections 1 are spaced closer together,

atcnt o hdiildil $31 the magnetic forces tending to move the sections l2 apart become greater.

The number of folds in the the desired physical size of of the ribbon Zlti. ribbon to is folded parallel sections 12.

Disposed between the folded sections 12 of the ribbon are spacers or strips 1d of hard, heat resistant insulating material, such as phenoli impregnated canvas. The spacers lid, in addition to serving as insulation between the adiacent sections 12 serve as backing members for the sections 12.

in order to increase the electrical creep path between the section 32, the spacers are made wider than the ribbon and are transversely staggered as shown in FIGURE 3. The spacers M are made longer than the sections 12 and, as shown in FIGURE 2, end spacers to, which are short lengths of the insulatin material, are disposed between the outwardly extending longitudinal ends of the spacers 16 in abutting relation 1 'p with the interconnecting portions 13, whereby the interconnecting portions 13 are retained in position.

Electrical connections are made to the of the ribbon lb through a pair of spaced apart L-shaped terminal blocks and of conductive material, the corresponding legs of which are in parallel relationship. The terrni and 2 2 are insulated from each other by a sheet of suitable insulation, such as silicone rubber. The sheet 24 is extended beyond the boundaries or" the terminal and between spacers id to increase the electrical creep path between the terminals 2t; and 22. The ends of the ribbon it? are connected to the inwardly facing ma .-al portions of the shorter legs 26 and 2% of the terminals and 22, respectively, by suitable means, such as hard solder.

Means are provided on each of the longer legs 34 and 32 of the terminals 2% and 22, respectively, for attaching electrical cable (not shown) thereto. in the illustrated embodiment, the attaching means for terminal 2e includes an internally threaded tubular conduit 3d suitably connected at right angles to the longer leg 35) thereof. The attaching means for terminal 22 includes an internally threaded tubular conduit 36 which in suitably connected to an ofiset portion 33 of the leg 32 of the terminal 22.

Elongated rectangular elements or side forms and d2; of hard, heat resistant insulating material such as phenolic impregnated paper are provided on the outwardly facing surfaces of the folded ribbon ill. The ends of the elements and d2 are provided with suitable recesses ed and 46, which respectively receive the shorter legs 26 and 28 of the terminals 2d and 2.2. The elements it and are respectively attached to the terminals 2t) and 22 by suitable means such as screws 3 threadedly engaged with the terminals and 22.

When a high current is passed through the ribbon lit, the magnetic pr ssure forces the ribbon into any voids existing adjacent the ribbon id, that is, voids in the spacers id and lo, and in the side forms 4d and E2, and results in tearing of the ribbon ltl. In the illustrated embodiment, such voids occur at the separations (indicated by the reference numeral 5 existing between the ends or the terminals 2d and 22 and the recesses 44 and 45. To prevent tearing of the ribbon lit), the voids 5t? are filled with a hard insulating material, such as epoxy resin.

The sections 12 of the ribbon lit, the spacers and lie and the side forms as and 42 are clamped together by suitable clamping means 51 which in the illustrated embodiment, includes a plurality of transversely extend ribbon it) depends upon the resistor, and the length In the illustrated embodiment, the seven times to form four sets of ing, longitudinally spaced apart clamps. Each of the clamps 52 includes a pair of steel channels 54 and 56,

respectively disposed on the outwardly facing surfaces of the side forms id and 42. The channels 54 and as are connected together by bolts 53 and so extending through the ends of the channels 54 and 56. In the illustrated embodiment, each of the bolts 58 and so is provided with a nut 62 and washer 64-.

When a high current is passed through the ribbon 10, the ribbon tends to lengthen due to the heat developed therein by the current. This lengthening of the ribbon lid, if not restrained, will eventually tear the ribbon apart. To prevent such lengthening the clamps $2 are spaced along the side forms as and at such a distance and are maintained under such a pressure by the bolts 58 and do that the frictional force on the ribbon lid is greater than the force due to thermal expansion of the ribbon The necessary clamping pressure may be determined by first calculating the temperature rise of the ribbon it) assuming that all the energy supplied to the resistor is dissipated in the ribbon in a very short time. Using this temperature rise, the elongation of the ribbon may be calculated, and then using Youngs modulus of elasticity for the material of the ribbon, the lengthwise pressure may be calculated. The lengthwise force is then found by multiplying the pressure by the cross-sectional area of the ribbon. The necessary clamping force is then calculated by dividing the lengthwise force in the ribbon by the coefiicient of friction between the material of the ribbon and the material of the spacers l4 and 16 and of the side forms 40 and 42.

in one embodiment of a low inductance resistor, the ribbon is made of an alloy of 43% nickel and the balance copper. The ribbon is 1.125 inches Wide and 0.004 inch thick and is approximately 12 feet long. The ribbon is folded 7 times and inch phenolic impregnated canvas spacers are disposed between the folds of the ribbon.

The side forms are /2 inch thick phenolic impregnated paper. The unit is clamped together by /2 x 1 inch steel channels, which are tied together by A" bolts. The channels are spaced approximately every 2 inches along the length of the resistor and the clamping force on the ribbon is equal to about 100 lbs. per bolt.

The above resistor has an inductance of less than 0.05 microhenry and a resistance of 0.6 ohm. The resistor may be pulsed over 3,000 times at one minute intervals with 20,000 ampere pulses which last for 5 microseconds Without adversely affecting the structure of the resistor. The temperature inside the resistor during such an operation is about 300 F. The temperature at the surface of the resistor is about 200 F.

As can be seen from the above a resistor construction is provided which has a very low inductance and a high current capacity. The reason the resistor can Withstand high cur-rents is that the clamping pressure on the side forms is large enough so that the frictional force on the ibbon is greater than the forces due to thermal expan sion of the ribbon. Another reason is that all voids into which the magnetic pressure would force and tear the ribbon are filled with hard material. Moreover, the folded sections of the ribbon extend coextensively and therefore the sidewise magnetic pressure on the ribbon is small.

Various changes and modifications may be made in the above described low inductance, high current resistor without departing from the spirit or scope of thepresent invention. Various features of the invention are set forth in the accompanying claims.

What is claimed is:

l. A low inductance resistor having a pulsed current capacity greater than 1000 amperes, which resistor comprises a strip of resistive material folded back upon itself in uniform fashion to provide a plurality of coextensive parallelly arranged planar segments that are spaced apart and joined by folded end sections, a plurality of insulating spacers, one each of said insulating spacers being disposed between each pair of adjacent ones of said coextensive parallelly arranged planar segments of said strip, a plurality of insulating elements, one each of said insulating elements covering the exposed surface of each of said folded end sections of said strip, a pair of terminal blocks, a first of said terminal blocks being electrically connected to one end of said strip of resistive material and a second of said terminal blocks being electrically connected to the other end of said strip, a pair of side forms positioned along oppositely disposed planar segments of said strip of resistive material and adjacent the connection of the ends of said strip with said terminal blocks, a plurality of clamps spaced along the longitudinal axis of each of said side forms for clamping said side forms, spacers, insulating elements, and planar sections of said strip of resistive material together with a pressure such that the frictional force on said planar segments and folded end sections of said strip is greater than the force imparted thereto due to thermal expansion resulting from the passage of current therethrcugh, and insulating means for filling voids existing in the clamped together structure to prevent the possibility of said strip of resistive material being severed due to the action of magnetic pressure forces thereon while current is passed therethrough from said first terminal to said second terminal.

2. A low inductance resistor having a pulsed current capacity greater than 1000 amperes, which resistor comprises a strip of resistive material folded back upon itself in uniform fashion to provide a plurality of coextensive parallelly arranged planar segments that are spaced apart and joined by folded end sections, a plurality of insulating spacers, one each of said insulating spacers being disposed between each pair of adjacent ones of saidcoextensive parailelly arranged planar segments of said strip and extending beyond the edges thereof, a plurality of insulating elements, one each of said insulating elements covering the exposed surface of each of said folded end sections of said strip, a pair of terminal blocks, a first of said terminal blocks being electrically connected to one end of said strip of resistive material and a second of said terminal blocks being electrically connected to the other end of said strip, a pair of side forms positioned along oppositely disposed planar segments of said strip of resistive material and adjacent the connection of the ends of said strip with said terminal blocks, a plurality of clamps spaced along the longitudinal axis of each of said side forms for clamping said side forms, spacers, insulating elements, and planar sections of said strip of resistive material together with a pressure such that the frictional force on said planar segments and folded end sections of said strip is greater than the force imparted thereto due to thermal expansion resulting from the passage of current therethrough, and insulating means for filling voids existing in the clamped together structure to prevent the possibility of said strip of resistive mate rial being severed due to the action of magnetic pressure forces thereon while current is passed therethrough from said first terminal to said second terminal.

References fitted in the file of this patent UNITED STATES iATENTS Little Oct. 16, 1928 

1. A LOW INDUCTANCE RESISTOR HAVING A PULSED CURRENT CAPACITY GREATER THAN 1000 AMPERES, WHICH RESISTOR COMPRISES A STRIP OF RESISTIVE MATERIAL FOLDED BACK UPON ITSELF IN UNIFORM FASHION TO PROVIDE A PLURALITY OF COEXTENSIVE PARALLELLY ARRANGED PLANAR SEGMENTS THAT ARE SPACED APART AND JOINED BY FOLDED END SECTIONS, A PLURALITY OF INSULATING SPACERS, ONE EACH OF SAID INSULATING SPACERS BEING DISPOSED BETWEEN EACH PAIR OF ADJACENT ONES OF SAID COEXTENSIVE PARALLELLY ARRANGED PLANAR SEGMENTS OF SAID STRIP, A PLURALITY OF INSULATING ELEMENTS, ONE EACH OF SAID INSULATING ELEMENTS COVERING THE EXPOSED SURFACE OF EACH OF SAID FOLDED END SECTIONS OF SAID STRIP, A PAIR OF TERMINAL BLOCKS, A FIRST OF SAID TERMINAL BLOCKS BEING ELECTRICALLY CONNECTED TO ONE END OF SAID STRIP OF RESISTIVE MATERIAL AND A SECOND OF SAID TERMINAL BLOCKS BEING ELECTRICALLY CONNECTED TO THE OTHER END OF SAID STRIP, A PAIR OF SIDE FORMS POSITIONED ALONG OPPOSITELY DISPOSED PLANAR SEGMENTS OF SAID STRIP OF RESISTIVE MATERIAL AND ADJACENT THE CONNECTION OF THE ENDS OF SAID STRIP WITH SAID TERMINAL BLOCKS, A PLURALITY OF CLAMPS SPACED ALONG THE LONGITUDINAL AXIS OF EACH OF SAID SIDE FORMS FOR CLAMPING SAID SIDE FORMS, SPACERS, INSULATING ELEMENTS, AND PLANAR SECTIONS OF SAID STRIP OF RESISTIVE MATERIAL TOGETHER WITH A PRESSURE SUCH THAT THE FRICTIONAL FORCE ON SAID PLANAR SEGMENTS AND FOLDED END SECTIONS OF SAID STIRP IS GREATER THAN THE FORCE IMPARTED THERETO DUE TO THERMAL EXPANSION RESULTING FROM THE PASSAGE OF CURRENT THERETHROUGH, AND INSULATING MEANS FOR FILLING VOIDS EXISTING IN THE CLAMPED TOGETHER STRUCTURE TO PREVENT THE POSSIBILITY OF SAID STRIP OF RESISTIVE MATERIAL BEING SEVERED DUE TO THE ACTION OF MAGNETIC PRESSURE FORCES THEREON WHILE CURRENT IS PASSED THERETHROUGH FROM SAID FIRST TERMINAL TO SAID SECOND TERMINAL. 